Precision current Sensing? (100s of microamps)
Precision current Sensing? (100s of microamps)
(OP)
Hi all,
I'm trying to integrate some current sensing into a project. I've got about 20 shunt resistors that I want to use to measure current into various portions of my circuitry.
Now, my plan is to create a daughterboard that can be attached to the mainboard to measure these currents. The design is pretty crowded right now, with a lot of digital stuff going on. At most, I can use 2 extra layers for my shunt resistor monitoring.
I want to use a MAX9934 to convert the voltages across the shunt resistors to single ended voltages, and run those to the daughterboard through a connector.
I'm going to use my two layer allowance in the following way: first = power plane + MAX9934 outputs, second = analog ground.
Finally, on the daughterboard, I want a high - resolution, multi channel ADC + uC.
Any pitfalls I should look for? I want to be able to measure both sub-milliamp to 100s of milliamps. This results in looking for 100's of microvolts to ~100 millivolts, which is quite a large range...
Any recommendations on ADC / practicality?
I'm trying to integrate some current sensing into a project. I've got about 20 shunt resistors that I want to use to measure current into various portions of my circuitry.
Now, my plan is to create a daughterboard that can be attached to the mainboard to measure these currents. The design is pretty crowded right now, with a lot of digital stuff going on. At most, I can use 2 extra layers for my shunt resistor monitoring.
I want to use a MAX9934 to convert the voltages across the shunt resistors to single ended voltages, and run those to the daughterboard through a connector.
I'm going to use my two layer allowance in the following way: first = power plane + MAX9934 outputs, second = analog ground.
Finally, on the daughterboard, I want a high - resolution, multi channel ADC + uC.
Any pitfalls I should look for? I want to be able to measure both sub-milliamp to 100s of milliamps. This results in looking for 100's of microvolts to ~100 millivolts, which is quite a large range...
Any recommendations on ADC / practicality?





RE: Precision current Sensing? (100s of microamps)
HTH
RE: Precision current Sensing? (100s of microamps)
By the way, why doesn't the datasheet speak in terms of gain, rather than saying "25uA/mV" or whatever? Since the input is a voltage sense, not current...
RE: Precision current Sensing? (100s of microamps)
I don't necessarily see that your gain choice will result in the dynamic range you claim as a requirement. Moreover, the high-gain device has a poorer frequency response, and poorer common-mode rejection ratio.
TTFN
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RE: Precision current Sensing? (100s of microamps)
Or flip that and have the shunts on the daughter board.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Precision current Sensing? (100s of microamps)
I didn't see that table.. thanks.
Can I use that amp as a current output device (rather than sticking the resistor on the output), and connect it to current-input ADC, such as: http://www.ti.com/lit/ds/symlink/ddc232.pdf ?
What are the pro/cons of doing that versus using it in that manner versus described in the datasheet (their example of using a 10k res, output hooked to SAR ADC)?
RE: Precision current Sensing? (100s of microamps)
Ok, so what is the correct way of doing it?
My constraints on Rsense are:
A) Power Consumption
B) Rail tolerance
C) Max / Min Vsense scale -> ADC to maximize dynamic range.
I had assumed it would be better to crank the gain up on the amp, as long as I didn't saturate the ADC.
Are you saying that for (say) a vsense of 100uV, it's better to have a gain of (say) 50 at the amp, rather than 250 in terms of measurement accuracy?
I understand for the case where I have flexibility for Rsense, that I should probably use the largest value I can, and use a smaller gain.
Assuming I use a 16 bit ADC, with Vref = 500mV, I will get 0.0076mV/count. That gives me 327 counts for 0.1mA over a .1ohm res, but only 65 counts at gain = 50.
My amps are going to be right at the sense res, so I figure that there mode be too much induced noise between the amp and resistor. On the other hand, the output of the amp will have to travel across a noisy board to get to the ADC. Doesn't that suggest I should sacrifice CMRR for higher gain?
RE: Precision current Sensing? (100s of microamps)
I think increasing the gain is a good idea. The key is to use a low offset and low offset drift amplifier. Some amplifiers called “zero drift” use an internal calibration circuit to correct for offset and drift. Below are links to a good article series on the topic of current monitoring. It includes info on many of the tradeoffs, error sources, and PCB layout stuff. I hope this helps.
http://www.eetimes.com/design/industrial-control/4...
http://www.eetimes.com/design/industrial-control/4...
http://www.eetimes.com/design/industrial-control/4...
http://www.eetimes.com/design/industrial-control/4...